In connection with wavelength division multiplexed (WDM) optical communication systems and other uses, there is a need for inexpensive, efficient means to effect wavelength dependent processing of optical signals. Examples of wavelength-dependent processing include, for example, wavelength selective optical switching, e.g., for routing individual wavelength channels in WDM systems, and wavelength-specific power attenuation, which can be useful for gain equalization in a WDM system.
A typical approach to wavelength dependent processing of optical signals involves first separating, or demultiplexing, individual wavelength bands as independent channels, and then processing the information from each channel separately. The processed channel information may be multiplexed back into a single transmission path, such as a single optical fiber, or separately distributed, or manipulated in various other ways.
A disadvantage of conventional demultiplexer- and multiplexer-based devices is that insertion losses are generally high, often as high as 4 –5 dB per stage. Another disadvantage is that devices relying on conventional demultiplexer geometry tend to be bulky. The systems also can be expensive.
Various switching techniques likewise have been implemented, each having its particular disadvantages. In one technique, for example, an optical signal is converted to the electrical domain to perform switching operations and then converted back to the optical domain for continued transmission or processing. This technique is expensive and is data format dependent.
There is a need for methods and apparatus capable of wavelength dependent processing of optical signals, wherein disadvantages associated with conventional multiplexer and demultiplexers, including losses, undue expense, bulky geometries, cross talk and similar problems, are substantially alleviated.
As applied according to the present invention, one way to facilitate selective processing of light, comprises imparting distinct polarization properties to one or more portions of the light that is to be processed differently from other portions (e.g., routed, attenuated, etc.). This can involve alteration of the polarization properties of an optical input signal as a function of wavelength, to selectively mark one or more wavelengths for particular processing. The polarization properties of an optical signal in that case can provide a parameter that permits components present in the optical signal to be discriminated or treated differently. For example, differently polarized signals can be routed along different signal paths by a polarization beam splitter, where they might be further processed in different ways.
Such polarization encoding can be useful in 7 various ways, for example to correct for distortion by adjusting power levels as a function of wavelength, to switch wavelength bands on and off or to route different wavelengths along different signal paths, etc. However, it is not readily apparent how wavelength specific polarization should be imparted in an efficient manner. The process would seem to require first separating the input signal into individual wavelength bands that are coupled to distinct transmission paths (i.e., demultiplexing), subjecting the separated individual wavelength bands to different processes, for example with some being subjected to a particular polarization alteration while others are not, and then recombining or multiplexing the differently-processed wavelengths. It is still less apparent how such wavelength band specific polarization could or should be imparted arbitrarily and changeably, whereby specific bands could be chosen for attenuation or switching, etc., on the fly.
One can understand some of the difficulties by considering a situation in which a signal containing a plurality of distinct wavelength bands passes through an optical device such as a waveplate. Assuming that all the light that enters the device also emerges, it would seem that some additional mechanism is needed for any wavelength-selective effects. This is particularly true if the effects are to be controllable, or if any substantial channel separation is desired, because all the portions of the input light that pass through are presumably equally or nearly-equally affected by the device. The wavelength dependent phase retardation might provide a basis for obtaining different effects as a function of wavelength, but any difference is likely to be minimal for closely adjacent wavelengths. Single waveplates would seem to have limited application. A series of stacked elements such as birefringent elements might affect particular wavelength bands differently from others, but this still does not seem a good candidate for use to arbitrarily manipulate selected wavelength channels or groups of channels. See, for example, Harris, Amman and Chang, “Optical Network Synthesis using Birefringent Crystals. I. Synthesis of Lossless Networks of Equal-Length Crystals,” and Amman and Chang, “Optical Network Synthesis using Birefringent Crystals. II. Synthesis of Networks Containing One Crystal, Optical Compensator, and Polarizer per Stage.”
A fixed stack of birefringent elements has been used in conjunction with a digital polarization converter to allow some control over the transfer function of an optical device. In particular, stacked birefringent elements have been tried as a solution to all-optical switching based on polarization as shown, for example, in U.S. Pat. No. 5,694,233 to Wu, et al. (“the '233 patent”).
There remains a need for a practical and workable device that can selectively and controllably change a polarization state of arbitrarily selected channels out of a plurality of channels, thereby allowing various further processing of an optical signal, among other useful results.